Without limiting the scope of the invention, its background is described in connection with high density metal-oxide-semiconductor (MOS) devices, and more particularly, to a high density metal-oxide-semiconductor structure which has metal silicide regions in the doped regions and metal gates.
Since the invention of the integrated circuit, work has been done to increase the number of components per unit of chip area and to improve device performance. The first integrated circuits were bipolar devices and used the junction isolation technique. However, as the demand for smaller and smaller devices increased, new technologies were developed which had higher packing density than bipolar devices. The self-isolating metal-oxide-semiconductor devices have now substantially replaced bipolar devices where very high packing density is required, such as in memories and microprocessors. As the demand for faster, higher density metal-oxide-semiconductor devices continues, improvements in fabrication technology are necessary to keep pace with the demand.
In the manufacture of very high density and high performance MOS VLSI devices such as the RISC microprocessors and 4-Megabit static RAMS (SRAMS), the electrical resistance of small-geometry elements has generally necessitated the use of a silicide cladding for polysilicon and source/drain regions. A self-aligned silicide, or "salicide" process is typically used to provide this cladding, and may be combined with a local interconnect to achieve smaller SRAM cell size. See U.S. Pat. No. 4,821,085 issued to Haken et al. on Apr. 11, 1989. Smaller SRAM cell size can also be achieved by using a polysilicon resistor load (4 transistor/2 resistor cell) or a thin film transistor (TFT) load device.
Heretofore, in this field, TFT devices are normally formed by using one layer of polysilicon for the transistor body and another (overlying or underlying) layer of polysilicon for the transistor gate, with a gate oxide in between. This approach places severe constraints on the process in terms of low temperature processing and hinders the use of silicides to lower the resistance of the polysilicon and the source/drain regions.